the research described here is intended to investigate the involvement of brain mast cells in the reproductive functions of male Syrian hamsters. The hamster is an attractive model to investigate the role of gonadotropin releasing hormone immunopositive (GnRH+) mast cells because the number of these cells in the brain is affected by the reproductive status of the animal (i.e., more cells are seen in photostimulated, reproductively active males). A similar correlation between cell number and reproductive competence has also been seen in doves and may be a common feature in vertebrates. The aims of this study are to (1) Confirm the phenotype of the GnRH+ non-neuronal cells as mast cells using electron microscopy, (2) Determine if GnRH+ mast cell induction in the brain is steroid-independent, and (3) Determine the relevant socio-sexual conditions under which GnRH+ mast cells migrate into the brain.